Measurement of the repulsive force between polyelectrolyte molecules in ionic solution: hydration forces between parallel DNA double helices.

We have measured the repulsive force between B-form double helices in parallel packed arrays of polymer-condensed DNA in the presence of 0.005-1.0 M ionic solutions. Molecular repulsion is consistently exponential with a 2.5-3.5 A decay distance, when the separation between DNA surfaces is 5-15 A. Only weakly dependent on ionic strength and independent of molecular size, this intermolecular repulsion does not obey the predictions of electrostatic double-layer theory. Rather, it strongly resembles the "hydration force" first recognized and quantified between phospholipid bilayers. Only beyond 15 A separation between molecules is there evidence of electrostatic double-layer forces. The quantitative failure of electrostatic double-layer theory seen here must gravely affect accepted analyses of other polyelectrolyte systems. Because the packing of condensed DNA resembles the spacings of DNA in many bacteriophages, our results permit estimation of the "DNA pressure" in phage heads.